U.S. Pat. No. 4,528,848 discloses an apparatus of such a type of a loop conveyor, comprising a housing including a side wall having an essentially cylindrical inner surface, an upper and a lower face wall provided with charging and discharging ports respectively and a rotor mounted in and sealed to the housing for rotation about a vertical axis. The rotor is provided with a plurality of laterally closed pockets and has a peripheral cylindrical surface forming an annular interspace with the inner cylindrical surface of the side wall of the housing. Pourable material filled into the rotor through the charging port is conveyed through a measuring path and then discharging through the discharge port. With the known apparatus, the lower and upper face walls of the housing are in engagement with the upper and lower faces of the rotor which results in a noticeable friction between the engaging parts. Also, despite the seals provided, some of very fine particles of the pourable material, for instance coal dust, may penetrate out of the pockets into the interior of the rotor or into the interspace between the inner cylindrical surface of the housing and the peripheral cylindrical surface of the rotor as well as in free spaces in the central portion of the rotor and its bearing system.